graphenegraphene-flagship.eu/lists/deliverables/fp7 d15.3 definition of... · graphene d15.3 15...

Project funded by the European Commission under grant agreement n°604391

GRAPHENE Graphene-Based Revolutions in ICT and Beyond

Combination of CP and CSA

WP15 Research Management

Deliverable 15.3, formerly Deliverable 11.3

“Definition of vocabulary, characterization methods and materials template”

Main Author(s): Matthieu Paillet, UM, Norbert Fabricius, KIT

Due date of deliverable: M30

Actual submission date: M30

Dissemination level: PU

GRAPHENE D15.3 15 March 2016 2 / 21

LIST OF CONTRIBUTORS

Partner Acronym Partner Name Name of the contact

78 UM Université de Montpellier Matthieu Paillet

78 UM Université de Montpellier Jean-Roch Huntzinger

78 UM Université de Montpellier Maxime Bayle

78 UM Université de Montpellier Ahmed-Azmi Zahab

78 UM Université de Montpellier Jean-Louis Sauvajol

78 UM Université de Montpellier Périne Landois

78 UM Université de Montpellier Benoit Jouault

78 UM Université de Montpellier Antoine Tiberj

79 UNIZAR Universidad de Zaragoza Raul Arenal

79 UNIZAR Universidad de Zaragoza Luc Lajaunie

80 LNE Laboratoire National de Métrologie

et d’Essais

Félicien Schopfer

80 LNE Laboratoire National de Métrologie

et d’Essais

Sébastien Ducourtieux

81 HORIBA HORIBA Jobin Yvon SAS Bertrand Dutertre

82 UNamur University of Namur Luc Henrard

82 UNamur University of Namur Jean-François Colomer

82 UNamur University of Namur Nicolas Reckinger

82 UNamur University of Namur Alexandre Felten

83 KIT Karlsruhe Institute of Technology Norbert Fabricius


TABLE OF CONTENTS Deliverable Summary ...........................................................................................................................4 Standardisation ....................................................................................................................................5

Linking the Graphene Flagship standardization activities to IEC/ISO ................................................5 Terminology .....................................................................................................................................6 Materials template ...........................................................................................................................6 Characterisation methods ................................................................................................................6 Outlook ............................................................................................................................................7

KCC 02 Graphene - Evaluation of the number of layers........................................................................8 Measurement protocol .....................................................................................................................9 Review of criteria based on Raman and optical contrast ...................................................................9 Number of layers specification on soda lime glass and SiO2/Si substrates using AGNorm and optical

contrast ......................................................................................................................................... 11 References ........................................................................................................................................ 14 Annex ................................................................................................................................................ 15

Raman, reflection, transmission setup ............................................................................................ 15 Detail of the maps used for Figure 3a of the main text .................................................................... 17 Measurements of SiO2 thickness on oxidised silicon wafers ........................................................... 18 Other substrates ............................................................................................................................ 19 Evaluation of the number of layers by X-ray photoemission spectroscopy ...................................... 21


Deliverable Summary

One of the key success factors for the production of products based on Graphene Related Materials

(GRM) is the availability of material with a consistent quality as well as reliable fabrication processes.

This requires that all stages in the fabrication process are controlled by sophisticated technical quality

management methods with documented material specifications, test methods and standard operating

procedures. Furthermore, the safe and reliable use of the products requires that the manufacturer has

a reliability assessment system in place. All this is only possible if the key control characteristics and

the related measurement methods for the fabrication process are known and clearly defined in global

standards. As the Flagship intends to drive GRM technologies forward from fundamental research to

commercial fabrication, standardisation is an important and cross work package activity.

This deliverable is the first step of a process which will accompany the scientific and technical

development until the completion of the whole mission in 2023. It is also the first step to integrate

standardisation related activities performed in all the scientific work packages into a flagship

standardization concept. Nevertheless, as GRM are the focus of similar activities everywhere in the

world, standardisation within the Graphene Flagship needs to be linked to global standardization

activities to ensure consistency. Therefore communication and cooperation with the relevant

Standards Development Organisations are central to this deliverable. In this context, the Technical

Committee 113 (TC 113) of the International Electrotechnical Commission (IEC) provides the interface

to other technical committees of IEC and of the International Organisation for Standardization (ISO).

The focus of the task is on documents dealing with the definition of vocabulary, characterisation

methods and materials template – activities that provide the basis for further standardisation. In

international standardisation, these type of documents are referred to: Blank Detail Specification

(BDS) which is used for all types of material templates listing the material properties and Key Control

Characteristics (KCC) for the material properties itself.

This report presents the status reached regarding

• the cross work package collaboration level (GFSC, Graphene Flagship Standardisation Committee),

• the vocabulary (ISO/TS 80004-13), • the harmonisation between initial Flagship-internal material datasheets and the BDS in

international standardisation and their database representation (Materials database) • the status of other standardization activities established in the GFSC • the measurement standard for the KCC “Number of layers”

Our approach combines simultaneous Raman spectroscopy (G band integrated intensity) and laser

optical contrast mapping to determine the number of layers of graphene on given substrates ( glass

and on oxidized silicon, with an oxide thickness of 90 ±5 nm). This method is limited to high quality

graphene/few layer graphene with small defects density and low residue and for a number of layers

up to 5. Here, we report the details of the characterisation methods, summarise the results obtained

and present how the criteria and their domain of validity or limitations were established.


Standardisation In light of the international standardisation activities on graphene within IEC and ISO, it is most important

that related activities in the Flagship not be in contradiction with upcoming standards in IEC and ISO. It

is also important that the Flagship is seen by the international standardisation bodies as an important

stakeholder to ensure that the Flagship interests, that means EU-interests regarding graphene

technologies, are reflected in future standards. The degree of Flagship participation in international

standardisation will have a strong impact on the overall Flagship target to support the EU economy. One

of the main targets was therefore the implementation of processes that enable the interaction between

the involved standardisation bodies and the Flagship and increase of the visibility of the Flagship in the

standardisation community. The following results relating to D15.3 were achieved.

Linking the Graphene Flagship standardization activities to IEC/ISO

Within WP11, subsequently shifted to WP15, the Graphene Flagship Standardisation Committee (GFSC)

was established. All other WPs were invited to participate in the GFSC and 60 scientists are currently

listed as members of the GFSC (See figure 1). The GFSC ensures that the important documents

prepared by IEC and ISO are available on the Flagship intranet site “Onboard” under Standardisation.

Figure 1: GFSC membership versus Flagship work packages.

A workshop (WS) was arranged within the European standardisation body CENELEC to allow the

publication of official European standardisation documents, so called CENELEC Workshop Agreements

(CWA). Involving an official standardisation body into the standardisation activities of the Flagship will

give more weight to the published documents and allows for an easier and quicker transfer of these

documents onto the IEC level. Some of the GFSC members already participate in name of their

organisations in this WS. These organisations will be listed as authors of the upcoming CWAs. The

formal difference between the GFSC and the CENELEC WS is that the CENELEC WS is generally open

for non-Flagship members, whereas the GFSC is covered by the Flagship partners.

A liaison by expert participation was established with IEC/TC 113. As IEC/TC 113 is the leading

standardisation committee in the graphene area, the Flagship has now nearly the same rights as regular

country members of the IEC, except explicit voting on documents. Voting is possible over the national

committees of the countries from which the GFSC members come from. It is worth noting here that via

IEC/TC 113 the Flagship participates also in standardisation projects led by ISO, since all ISO led

graphene projects are joint projects with IEC/TC 113. Any member of the GFSC can comment on the

IEC/ISO documents circulated on Onboard based via the liaison.


Terminology

The terminology part of the delivery is related to ISO/TC 80004-13 “Nanotechnologies - Vocabulary -

Part 13: Graphene and other two dimensional materials” (IEC/ISO joint project). This standard provides

a comprehensive terminology for graphene. The project leader is from NPL, a Flagship partner. The

current version of the document is a harmonised version, including input from GFSC members gathered

during the first face-to-face meeting of the committee. As new versions of the document are available in

IEC/ISO these will be circulated on Onboard so that the GFSC members can comment on it.

Materials template

Part of D11.3 deals with the "materials template". This term refers to an “empty” datasheet which lists all

properties needed to describe a certain material comprehensively. Datasheets for graphene materials

available from Flagship partners were collected at the project start. Based on this information, a common

electronic version of the datasheets was developed, the material database section of Onboard.

The international standardisation community in IEC, IEEE and ISO is currently working on a material

template for graphene as well, the so-called Blank Detail Specification (BDS) IEC 62565-3-1. This is a

central document for all graphene standardisation as it does not only provide a comprehensive list of all

Key Control Characteristics (KCC) but will eventually list the respective measurement methods as well.

Therefore, it serves also as a roadmap for the standardisation activities indicating where measurement

methods still need to be defined. To ensure consistency between international standardisation and

Flagship activities and achieve a common template for the description of GRM to be used throughout

the industry, the comprehensive list of KCCs for graphene in the BDS needs to be harmonised with the

MDB. During the ramp-up phase, the harmonisation activities, performed in the context of the

standardisation task, managed to achieve a consistency of 80%. Thus, these activities need to be

continued under Core 1.

Characterisation methods

Part of D15.3 is the development of a measurement standard for the KCC “Number of layers”. This

included the development of the characterisation method described in the next section as well as the

incorporation of the scientific results into a standard document agreed upon within the GFSC. This

document, KCC 02 Nanomanufacturing - Key control characteristics - Graphene - Determination of the

number of layers by Raman spectroscopy combined with optical reflection, is about to be distributed

within the GFSC and will be published as a CWA in 2016. Especially taking into account that 18 months

of project time is included in the technical development of the method, the document reached the current

status in approximately half the time compared to the international IEC/ISO process.

From the beginning, the GFSC tried to motivate researchers from other WPs to get involved in the

standardisation task. Worth noting in this context is especially the standardisation activity proposed by a

group from WP6 Spintronics. The document, KCC 01 Nanomanufacturing - Key control characteristics -

Graphene - Uniformity of strain variations analysed by Raman spectroscopy, reached the same status

as KCC 02 and will be published as a CWA in 2016 as well. Furthermore, it was adopted by IEC/TC 113

as a Proposed Work Item (IEC/PWI 62607-6-6) after an official international vote (Resolution 10-20 at


the IEC/TC 113 autumn meeting in Seoul in November 2015), which means that it will most probably be

upgraded to a full consensus international standard in the near future.

Outlook

The GFSC understands the standardisation task to be a continuous activity over the whole project

duration until 2023. Furthermore, the GFSC sees a growing impact of the standardisation task on the

project over time when the amount of applied development increases. Therefore it is important to explore

the Flagship regarding other activities, which may be subject to future standardisation and to relate this

to the international activities in IEC/ISO. The Table 1 lists all current standardisation projects on graphene

in the Flagship and IEC/ISO.

Number Title GFSC IEC/ISO

KCC 01 (IEC/TS 62607-6-6)

Nanomanufacturing - KCC - Graphene - Uniformity of strain variations in monolayer graphene analysed by Raman spectroscopy

Y Adopted from GFSC

KCC 02 Nanomanufacturing – KCC - Graphene - Determination of the number of layers by Raman spectroscopy combined with optical reflection

Y To be adopted from GFSC

KCC03 Nanomanufacturing - KCC - Graphene - Measurement of sheet resistance by the four-point probe method

Core 1 To be adopted from GFSC

KCC04 Nanomanufacturing - KCC - Graphene - Measurement of sheet resistance by the non-contact Eddy current method


KCC05 Nanomanufacturing - KCC - Graphene - Measurement of sheet resistance by terahertz time-domain conductance spectroscopy


KCC06 Nanomanufacturing - KCC - Graphene - Determination of the defect density of monolayer graphene analysed by Raman spectroscopy


RA01 Nanomanufacturing – Reliability assessment - Graphene – Bending test for graphene-based flexible electrodes


PWI Nanomanufacturing – Reliability assessment - Graphene – Basic reliability qualification for graphene layers, temperature and humidity

TBD TBD

ISO/TR 19733 (KR/US) Matrix of characterization and measurement methods for Graphene Comments Y

ISO/PWI 21356 (UK) Nanotechnologies -- Structural characterization of graphene Comments Y

ISO/TS 80004-13 (UK) Nanotechnologies - Vocabulary - Part 13: Graphene and other two-dimensional materials

Comments Y

IEC/TS 62565-3-1 (US) Nanomanufacturing - Material specifications - Part 3-1: Graphene - Blank detail specification

Material database

Y

IEC/TS 62565-3-2 (CA) Nanomanufacturing - Material specifications - Part 3-2: Graphene - Detail specification for nano-ink"

Comments Y

IEC/TS 62607-6-1 (KR) Nanomanufacturing - KCC - Part 6-1: Graphene - Electrical characterization

Comments Y

IEC/TS 62607-6-2 (KR) Nanomanufacturing - KCC - Part 6-2: Graphene - Evaluation of the number of layers of graphene

Co-Lead (TBD)

Y

IEC/TS 62607-6-3 (KR) Nanomanufacturing - KCC - Part 6-3: Graphene - Evaluation of the defect level in the graphene layer

Comments Y

IEC/TS 62607-6-4 (US) Nanomanufacturing - KCC - Part 6-4: Graphene - Non-contact conductance measurement using resonant cavity

N/A (PUB) Y

IEC/TS 62607-6-5 (KR) Nanomanufacturing – KCC – Part 6-5: Graphene – Sheet resistance and contact resistance measurement using the transmission line method

Comments Y

IEC/TS 62607-6-6 (KCC 01) (DE)

Nanomanufacturing – KCC – Part 6-6: Graphene – Uniformity of strain in graphene analyzed by Raman spectroscopy

Lead in Core1

Adopted from GFSC

Table 1: Projects in the GFSC and IEC/ISO: “Y” indicates an active project in the organization. “Core 1”

indicates that it is expected that the project will be approved and executed in the “Core 1”-phase of the

Flagship.


The IEC and GFSC/CENELEC activities are summarised in a joint roadmap. This ensures that the

standardisation activities are harmonised (See Figure 2).

Figure 2: Excerpt of the joint roadmap of IEC/TC 113 and GFSC/CENELEC

KCC 02 Graphene - Evaluation of the number of layers Different kinds of graphene samples produced, in the consortium and beyond, by the three major

methods (mechanical exfoliation from graphite, chemical vapour deposition and SiC sublimation) have

been investigated using a wide range of experimental tools (optical microscopy, spectral microreflection,

Raman spectroscopy, combined Raman/reflection/transmission, X-ray photoelectron microscopy,

scanning electron microscopy, atomic force microscopy and transmission electron microscopy). These

studies serve as the basis of the KCC 02 document which provides, at this stage, a standardised method

to determine the number of layers for graphene on glass and on oxidized silicon, with an oxide thickness

of 90 ±5 nm. This method is limited to high quality graphene (1LG) and few layer graphene (FLG) with

small defects density and low residue and for a number of layers up to 5. The number of layers (N) is

determined by the combination of two methods: Raman spectroscopy and optical reflection. Both

methods often enable to distinguish between graphene and multilayer graphene (MLG). However,

neither each method nor the combination of the two enable the determination of the number of layers in

all possible cases (especially regarding all possible stacking angles). Despite that, comparison of the

values deduced by both methods allows to discriminate if the determined number of layers is correct and

can be specified or not. Here, we report the details of the characterisation methods, summarise the

results obtained and present how the criteria and their domain of validity or limitations were established.


Measurement protocol

The home-made experimental setup used combines microRaman spectroscopy with microreflection and

microtransmission (Raman/R/T) measurements and was designed to optimise the signal to noise ratio

of measurements and its stability ((typical position drift is <200 nm over days). The impinging laser power

as well as the transmitted and reflected laser beam are simultaneously measured by photodiodes and

continuously monitored during Raman signal acquisition. This setup has been used to simultaneously

record Raman, optical contrast and extinction maps of the graphene/FLG samples with a typical yield of

100 000 measured points per day with a 1 mW incident laser power. The continuous incident laser power

monitoring allows to get rid of the laser power fluctuations’ impact on the measured quantities (Raman,

reflected and transmitted laser intensities). This experimental setup implemented by UM has served as

the basis for the prototype developed by HORIBA (deliverable 15.4). A home-made user-friendly Labview

application has also been developed for data treatment. It includes specific algorithms to subtract the

substrate’s Raman background, automated fitting procedures, data visualization and interpretation tools.

For the measurements, five laser wavelengths (457, 491, 532, 561 and 633 nm) and two microscope

objectives (100x numerical aperture (NA) 0.9 and 50x NA 0.5) have been used. To ensure that the

measurements are independent of the setup used, the measured Raman intensities are normalised to

HOPG and by the incident laser power. The Raman/R/T measurements have been complemented with

optical microscopy, spectral microreflection measurements (in the 400-800 nm range with a typical

analysed spot size of 1.5 µm²) and theoretical calculations.

Review of criteria based on Raman and optical contrast

Our approach, combining simultaneous Raman spectroscopy and laser optical contrast mapping,

enables to discuss the evolution of the Raman features in FLG for which the number of layers, N, is

corroborated by optical contrast.

2D-band based criteria: As reported in ref. [1], we examine the dependencies of the full width at half

maximum of the 2D band (Γ2D) and the ratio between 2D and G bands integrated intensities (A2D/AG) as

a function of N since these parameters have been commonly used in the literature as metrics to

distinguish 1LG and FLG: 1LG has been proposed to have the lowest Γ2D and highest A2D/AG than MLG.

During our systematic investigation, we evidence different and even opposite behaviours of these

features with N [1]. Our results are analysed as the consequences of different stacking order between

consecutive graphene layers. In agreement with published reports on twisted bilayer graphene (2LG),

we demonstrate that higher values of the A2D/AG ratio and narrower 2D bandwidths than those measured

on 1LG can be measured on twisted FLG. In terms of control characteristics, these results confirm that

neither A2D/AG nor Γ2D are valid criteria to identify 1LG or to count the number of layers in FLG. The

sensitivity of these quantities to doping or strain also impact their reliability. As a consequence, criteria

based on the 2D band have been ruled out.

G-band area based criterion: A more robust parameter to count the number of graphene layers is the G

band area or integrated intensity (AG). Since it relies on Raman intensity measurement, it is necessary

to define a reference for intensity normalisation in order to enable the comparison of results obtained on

different systems and in different laboratories. HOPG has been chosen as a reference since it is a well-

GRAPHENE D15.3 15 March 2016 10 / 21

defined, easy to purchase material. More importantly, when the HOPG G-band area (𝐴𝐴𝐺𝐺𝐻𝐻𝐻𝐻𝐻𝐻𝐺𝐺) is used for

intensity normalization, no additional corrections from spectrometer sensitivity are required. In the

following, 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 stands for the ratio between AG and 𝐴𝐴𝐺𝐺𝐻𝐻𝐻𝐻𝐻𝐻𝐺𝐺 measured in the exact same conditions. AG

has the advantage to enable to distinguish in all cases between 1LG and FLG, if the signal to noise ratio

is high enough. However, regarding the number of layers counting, two limitations related to the relative

orientation and stacking of the graphene layers exist. First, an intensity enhancement can occur due to

changes in the joint density of states, for given relative orientations of the layers [2]. Second, we found

a significant G-band intensity decrease (down to 70% of the one of equivalent Bernal stacked structures)

for some relative orientations (Figure 3a and ref. [3]). As an example, for 2LG and a laser wavelength of

532 nm, the optical resonance increases AG for twist angles in the range 10° to 16° (Figure 3b, bottom)

and AG is found lower than in Bernal 2LG for twist angles in the range 16° to 23° (not shown). These two

limitations circumvent the use of AG alone as metrics for counting the number of layers.

Optical contrast based criterion: The optical contrast in the visible, defined as the ratio between the laser

signal reflected by the sample and the laser signal reflected by the bare substrate minus one, has also

been proposed as a tool for counting graphene layers [4]. Indeed, the optical properties of MLG are, in

most of the cases, directly related to the number of layers. However, as illustrated for twisted 2LG on

Figure 3b (top), the optical contrast is also changing near optical resonances. In this case, this criterion

also leads to a wrong determination of the number of layers.

Moreover, both AG and optical contrast are strongly dependent on the nature of the substrate and on the

laser wavelength used [4, 5]. This implies that each substrate needs to be specifically studied and a large

set of experimental data is a prerequisite to validate theoretical predictions.

In conclusion, we propose a standard method for the specification of the number of layers based on the

combination of Raman spectroscopy (normalised G-band area) and optical reflection (optical contrast).

Both methods enable to distinguish unambiguously between single layer graphene and multilayer

graphene. However, neither each method nor the combination of the two enable a determination of the

number of layers for all possible stacking orientations. But importantly, since the two methods always

significantly disagree when they fail, the comparison of the values deduced by each method allows to

discriminate if the determined number of layers is correct and can be specified or not.

Figure 3: (a) Illustration of the case where 𝑨𝑨𝑮𝑮𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵 is found lower than in equivalent Bernal stacked systems: 3D bivariate histogram of 𝑨𝑨𝑮𝑮𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵 and laser optical contrast extracted from maps obtained on a sample made by CVD on copper and transferred on a 90 nm SiO2/Si substrate (Figure 1 of ref. [1] and annex). The open circles are data obtained on mechanically exfoliated samples on 90 nm SiO2/Si

(a) (b)

GRAPHENE D15.3 15 March 2016 11 / 21

substrates. (b) Illustration of the impact of an optical resonance for bilayer graphene (2L) deposited on 300 nm SiO2/Si substrate: (top panel, black points) optical contrast and (bottom panel, green points) AG relative to the AG of Bernal bilayer (Bernal2L) as a function of the twist angle between the two layers measured with a 532 nm laser. The blue dashed lines in the top panel represents optical contrast of graphene (1L) and Bernal 2 to 4 layers graphene (2L, 3L and 4L) as labelled on the figure.

Number of layers specification on soda lime glass and SiO2/Si substrates using

𝑨𝑨𝑮𝑮𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵 and optical contrast

Each point in the following graphs represents statistical analysis on thousands of measured points.

Mechanically exfoliated samples are measured as reference samples with a well-defined stacking

structure, low defects density and low residue and the other cases discussed above are taken into

account to define specification criteria. The error bars are set at +/- 3 sigma of the distribution of

measured values and also include the error of the measured reference (of the order of 2% for 𝐴𝐴𝐺𝐺𝐻𝐻𝐻𝐻𝐻𝐻𝐺𝐺).

Soda lime glass substrate

𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁, optical contrast and extinction as a function of number of layers measured on mechanically

exfoliated samples on soda lime glass using a 532 nm laser and 100x objective (NA 0.9) are presented

on Figure 4 and compared to theoretical calculations. The optical extinction is defined as one minus the

ratio between the laser signal transmitted through the sample and the laser signal transmitted through

the bare substrate.

Figure 4: (a) 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁, optical (b) contrast and (c) extinction as a function of the number of layers (N) measured on mechanically exfoliated samples on soda lime glass (blue dots) and predicted by theory (green lines). R stands for microreflection and OM for optical microscopy.

SiO2/Si substrates

Additional limitations

Silicon wafer suppliers specify usually the thermal oxide (SiO2) thickness at ±5%. Here, we have

measured the precise SiO2 thickness near the studied graphene/FLG flakes using spectral

microreflection. This method was validated using ellipsometry and TEM on specimens prepared by FIB

(see annex).

Since 90 and 300 nm are the most used SiO2 thicknesses as graphene substrate, we limit our report to

these two cases although others have been investigated. We established that:

(1) both AG and the optical contrast should be close to their maxima in the specified SiO2 thickness range

to ensure a good accuracy of the measurements,

(2) AG and the optical contrast values should exhibit weak variation with SiO2 thickness in these ranges,

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Data Model

AG/A

HO

PGG

N (from spectral R and OM)0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

Con

tras

t

N (from spectral R and OM)0 2 4 6 8 10

0.0

0.1

0.2

Extin

ctio

n

N (from spectral R and OM)

(a) (b) (c)

GRAPHENE D15.3 15 March 2016 12 / 21

(3) since the true numerical aperture of the microscope objective can vary for the incident laser light

depending on the illumination conditions which can be different from one setup to another, it is also

important that the measured values (AG and the optical contrast) have a weak dependence on NA.

As illustrated in the case of graphene on Figure 5, the above conditions are fulfilled for 90 ±5 nm SiO2

thicknesses and a laser wavelength of 532 nm but cannot be fulfilled for 300 ±15 nm SiO2 even by

changing the laser wavelength. As a consequence, the standardized method application is limited to

SiO2 on silicon with a SiO2 thickness of 90 ±5 nm and a laser wavelength of 532 nm.

Figure 5: (a) optical contrast and (b) 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 of graphene as a function of SiO2 thickness for a 532 nm laser and two microscope objectives (50x, NA 0.5 (black) and 100x NA 0.9 (red)). Black squares and red open dots correspond to measurements, black and red lines to theoretical calculations. The greyed regions highlight the 90 ±5 nm and 300 ±15 nm SiO2 thickness ranges. AGNorm and optical contrast as a function of number of layers measured on mechanically exfoliated

samples on 90 ±5 nm SiO2 on silicon using a 532 nm laser and 100x objective (N.A. 0.9) are presented on Figure 6. The results of a round robin test conducted in two different laboratories (UM and HORIBA) and using different spectrometers are also shown.

Figure 6: (a) 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 and (b) optical contrast as a function of number of layers between 1 and 5 on 90 nm ±5 nm SiO2 on Si. Open circles are experimental data (colour coded with the SiO2 thickness (nm) of the sample as displayed on the graph) and the solid black line is a polynomial fit (R²=0.9998 and 0.997). (c) Comparison of the results obtained at UM and at HORIBA for 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 as a function of number of layers.

Number of layers specification

The number of layers (N) is then estimated using NormGA and optical contrast (C ), for a 532 nm laser

and using a 100x objective with numerical aperture of 0.9, from the relations:

On soda lime glass: NG=7.16× +3.36× ( )2NormGA and NC=10.6×C -1.1× ( )2C

On 90 nm ± 5 nm SiO2 on Si: NG=1.05× +0.16× ( )2NormGA and NC=-5.74×C +4.61× ( )2C

0 50 100 150 200 250 300 350-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

300 +/- 15 nm

Opt

ical

con

trast

SiO2 thickness (nm)

Data (objective 50x, NA 0.5) Model (objective 50x, NA 0.5) Data (objective 100x, NA 0.9) Model (objective 100x, NA 0.9)

90 +/- 5 nm

0 50 100 150 200 250 300 3500.0

0.5

1.0

Data (objective 50x, NA 0.5) Model (objective 50x, NA 0.5) Data (objective 100x, NA 0.9) Model (objective 100x, NA 0.9)

ANor

mG

SiO2 thickness (nm)

90 +/- 5 nm 300 +/- 15 nm

0

1

2

3

0 1 2 3 4 5

Experimental data Fit

AN

orm

G

Number of layers

85

90

95

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0 1 2 3 4 5

Experimental data Fit

Number of Layers

Opt

ical

con

trast

85

90

95

(a) (b)

(a) (b) (c)

GRAPHENE D15.3 15 March 2016 13 / 21

The two values NG and NC are then compared using the graphic of Figure 7 where the black and grey

regions define in which cases N can be specified. The black regions correspond each to a given number

of layers between zero and five while the grey regions correspond to non-integer numbers of layers and

should be specified as: zero to one, one to two, etc. The cases where the method fails to give the true

number of layers, e.g. when NormGA or C are influenced by an optical resonance, will fall out of the

delimited region and N is specified as larger than one and but not more precisely attributed.

Figure 7: Reference chart for the number of layers specification using the values NG and NC determined from Norm

GA and optical contrast (C) respectively, see text.

Conclusions and outlook We propose a standard method for the specification of the number of layers of graphene on given

substrates (glass and on oxidized silicon, with an oxide thickness of 90 ±5 nm) based on the combination

of Raman spectroscopy (normalised G-band area) and optical reflection (optical contrast). This method

is limited to high quality graphene/few layer graphene with low defects density and low residue and for a

number of layers up to five. The number of layers can be specified in most, but not all, cases. Exceptions

have been established for both technics and justify their combined used to discriminate if the determined

number of layers is correct and can be specified or not. More generally, the limitations of the technics

related to the stacking (relative layers’ orientations) and substrate have been established and will serve

for future works.

Work regarding the characterisation of the number of layers on other substrates (SiO2/Si with different

SiO2 thicknesses, SiC, Cu…) and/or using other techniques (spectral microreflection, X-ray

photoemission spectroscopy, atomic force microscopy, transmission electron microscopy…) has already

started (some examples are given in annex) and will be continued in the Core 1 phase. The main goal is

to extend the domain of application of the standard in terms of substrates and range of the number of

layers.

0 1 2 3 4 50

1

2

3

4

5

3-4L

4-5L

2-3L

1-2L

0-1L

5L

4L

3L

2L

N

G

NC

1L0L

GRAPHENE D15.3 15 March 2016 14 / 21

References [1] Bayle, M., Reckinger, N., Huntzinger, J.-R., Felten, A., Bakaraki, A., Landois, P., Colomer, J.-F.,

Henrard, L., Zahab, A.-A., Sauvajol, J.-L. and Paillet, M. (2015), Dependence of the Raman spectrum characteristics on the number of layers and stacking orientation in few-layer graphene, Phys. Status Solidi B, 252(11), 2375-2379 and references therein.

[2] Wu, J.-B. et al. (2014), Resonant Raman spectroscopy of twisted multilayer graphene, Nat. Comm. 5, 5309 and references therein.

[3] Hwang, J.-S. et al. (2013), Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene, Nanotechnology 24, 015702.

[4] Casiraghi, C. et al. (2007), Rayleigh imaging of graphene and graphene layers, Nano Lett. 7, 2711. Blake, P. et al. (2007), Making graphene visible, Appl. Phys. Lett. 91, 063124. Gaskell, P.E. et al. (2009), Counting graphene layers on glass via optical reflection microscopy, Appl. Phys. Lett. 94, 143101.

[5] Klar, P. et al. (2013), Raman scattering efficiency of graphene, Phys Rev. B 87, 205435.

GRAPHENE D15.3 15 March 2016 15 / 21

Annex

Raman, reflection, transmission setup

The scheme of the setup is presented on Figure 8. Raman spectra were recorded using an Acton SP2500

spectrometer fitted with a Pylon CCD detector and a grating that enables the measurement of the full

spectrum in the range 1000-3000 cm-1 within a single acquisition (i.e. for a 532 nm laser, 600 grooves/mm

grating corresponding to ~2 cm−1 between each CCD pixel). Optimised focus conditions have been

checked for each measurement. The samples are mounted on a three-axis piezoelectric stage (Physik

Instrumente) to ensure the precise positioning and focusing of the laser spot. The laser power was

continuously measured by a calibrated photodiode put behind the beam splitter which enables to correct

the laser power fluctuations during the sample mapping. To perform simultaneously microreflection

(optical contrast) (resp. microtransmission (optical extinction)) measurements and Raman spectroscopy,

a low noise photodiode is placed on the path of the laser beam reflected by the edge filter located in front

of the spectrometer’s entrance slit (resp. under the sample). The whole experimental setup

(spectrometer, piezoelectric stage, photodiodes…) were controlled by a dedicated and home-made

Labview application. The acquisition time for each individual spectrum was adjusted to exceed a signal

to noise ratio of 50 for the G band (with a 532 nm laser, 1 mW on the sample and a 100x objective, the

requested acquisition time is 0.5 s (resp. 2 s) for graphene on 90 nm SiO2/Si (resp. glass)). The

experimental setup is fully enclosed to avoid any external perturbations. Together with its designed great

mechanical and laser pointing stabilities, this allows to almost cancel any XYZ drifts typically due to

ambient temperature changes. A home-made data analysis software was used to treat the ensemble of

the data (including normalisation of the intensity with regards to that of HOPG (reference sample),

corrections of the laser fluctuations, background subtraction, fitting of the bands…). An example of

extracted maps is shown on Figure 9.

Figure 8: Scheme of the Raman/reflection/transmission setup.

GRAPHENE D15.3 15 March 2016 16 / 21

Figure 9: (a) Optical microscopy image of a FLG sample deposited by micromechanical exfoliation on 89 nm SiO2/Si. The number of layers N is labelled as NL on the corresponding regions of the image and has be determined by spectral reflection, Raman and laser optical contrast. (b) G band integrated intensity normalised versus the one of HOPG map and (c) Optical contrast map extracted from a Raman/reflection mapping of the sample with a 532 nm laser and a 100x objective. (d) Profile along the white dotted line on (c) of the Norm

GA (left Y axis) and optical contrast (right Y axis).

20um

20um

0

4.5

0

-0.88Optical contrast

89 nm SiO2/Si(a) (b)

(c)

(d)

GRAPHENE D15.3 15 March 2016 17 / 21

Detail of the maps used for Figure 3a of the main text

Figure 10: (a) Optical image of a single layer graphene flake (1LG) including multilayer patches (darker regions) synthesised by CVD on copper and transferred on a 90 nm SiO2/Si substrate. Inset, zoom in the region delimited by the red square in the main image. (b-e) Different maps of the region highlighted by the red square in (a): (b) laser optical contrast, (c) width of the 2D band, (d) integrated intensity ratio of the 2D and G bands and (e) G band integrated intensity normalised versus the one of HOPG. The maps (b) and (e) have been used to plot Figure 3a of the main text.

10µm

0 50 t

0.5

4.2 (e) Norm

GA

GRAPHENE D15.3 15 March 2016 18 / 21

Measurements of SiO2 thickness on oxidised silicon wafers

In order to check the validity of the SiO2 thickness measurements on oxidised silicon wafers, we

compared the results obtained with the spectral microreflection method, with spectroscopic ellipsometry

measurements. Figure 11 shows an example of results obtained by both methods on a given SiO2/Si

sample: the spectral reflection gives a SiO2 thickness of 84.7 ±0.5 nm, in perfect agreement with

spectroscopic ellipsometry which confirms a thickness of 84.5 ±0.1 nm.

Figure 11: (a) Spectral microreflection measured on a SiO2/Si sample (the reference (R0) is a Si wafer with only a native oxide layer) as a function of the photon energy. The model (red curve) fits the experimental data (black dots) considering a SiO2 thickness of 84.7 ±0.5 nm. (b) Spectroscopic ellipsometry measurements on the same SiO2/Si sample. The model (solid lines) fits the experimental data (dots) considering a SiO2 thickness of 84.5 ±0.1 nm.

Figure 12 shows one of the Transmission Electron Microscopy (TEM) lamellas prepared by Focussed

Ion Beam (FIB). From this lamella, it has been possible, using a probe-corrected Cs TEM microscope

equipped with an energy dispersive X-ray spectrometer (EDS), to have access to the thickness of the

SiO2 substrate with a spatial resolution of 4-5 angstroms. Such sample preparations have also been

used to determine the number of graphene layers by TEM on samples deposited on SiO2/Si and

synthesised on SiC.

Figure 12: (a) optical microscopy and (b) atomic force microscopy images showing a MLG flake deposited by micromechanical exfoliation on a SiO2/Si substrate. (c)-(d) SEM images, showing the FIB prepared lamella. From this lamella, it has been possible to measure the SiO2 thickness via (e) scanning TEM images and (f) EDS measurements.

1.6 1.8 2.0 2.2 2.4 2.6 2.8

0.3

0.4

0.5

0.6

R/R

0

Energy (eV)

(a)

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

-1

0

1

2

3

4

5

6(b)

Tan(Ψ

)Energy (eV)

-1.0

-0.5

0.0

0.5

1.0

cos(∆)

GRAPHENE D15.3 15 March 2016 19 / 21

Other substrates

SiC substrates

The protocols for counting the number of layers of graphene grown on SiC using the combination of

Raman, reflection and transmission have been defined. This technique is routinely used at UM for

graphene/MLG samples characterisation at UM and data have been obtained on several samples.

Specific points have to be considered: 1) The 2nd order Raman spectrum of SiC falls in the graphene D

and G bands region, a specific algorithm for background subtraction has already been implemented

(Figure 13). 2) The possible presence of a buffer layer between the graphene and SiC has to be taken

into account in the graphene Raman spectrum fitting procedure. 3) Regarding extinction, limitations exist

depending on the nature of the sample back face and on the possibility of graphene growth on both

faces. During the breakout session of the Graphene Flagship General Assembly of October 2015, the

discussions highlighted the fact that the different groups working on graphene on SiC are using different

techniques for counting the number of layers. The necessity to set a workgroup to establish which

technique is best suited for standardisation was established and scheduled for the Core 1 phase of the

Graphene Flagship.

Figure 13: (a) Raman spectra of the SiC substrate (black), graphene on SiC (red) and background subtracted graphene spectrum (green) obtained using the developed algorithm. (b) Raman map of

NormGA and the corresponding histogram of a graphene sample grown by CVD on SiC.

Copper substrates

On copper, the laser wavelengths in the range 500-600 nm are found as not suitable due to the high

background signal in the Raman spectra of graphene/FLG in this case. Raman data have been collected

using 457, 473, 491 and 633 nm lasers and the values of 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 for graphene have been determined

(see Figure 14). The optical contrast and spectral reflection of graphene on copper are still under

analysis. Additional difficulty due to the possible presence of a (usually rough) copper oxide layer on

copper and/or underneath graphene have been pointed out. Indeed, even few nanometres of copper

oxide can strongly affect the optical properties of the copper substrate. The copper oxide thickness

strongly modulates the graphene/FLG Raman signal and optical contrast. For example, an amplification

of up to 5 times of the 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 of graphene has been measured using a 457 nm laser wavelength in

(a) (b)

GRAPHENE D15.3 15 March 2016 20 / 21

agreement with theoretical predictions. Interestingly, using a 457 nm laser, the copper oxide can be

sensitively detected by Raman spectroscopy at the same time as the graphene/FLG spectrum is

acquired (see Figure 15).

Figure 14: Histogram of 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 of graphene on copper without oxide obtained using a 457 nm laser wavelength and a 100x NA 0.9 objective. The experimental 𝐴𝐴𝐺𝐺𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 value is 4.5 (+/-1) % versus 5.1% predicted theoretically.

Figure 15: (a) optical image, (b) and (c) Raman integrated intensity maps of (b) the copper oxide signal and (c) the G-band. The integration ranges are depicted in brown (for (b)) and blue (for (a)) in the Raman spectra (e) and (f). (e) and (f) Representative Raman spectra recorded on a grey (resp. brown) region of (a), corresponding to copper without (resp. with) copper oxide.

(a) (b) (c)

(e) (f)

GRAPHENE D15.3 15 March 2016 21 / 21

Evaluation of the number of layers by X-ray photoemission spectroscopy

We used standard X-ray photoemission spectroscopy (XPS) to determine unambiguously the number of

graphene layers on Si/SiO2 substrates. The XPS technique is able to quantify the amount of elements

present on the surface of a material. Here, we are using this ability in order to evaluate the number of

layers of FLG by a quantification of the amount of carbon present at the surface of the substrate. As

graphene is a very thin material, this method is however highly dependent on the quantity of adventitious

carbon present on the graphene surface. For as-prepared sample (we used mechanical exfoliation of

graphite using a tape), we observe on Figure 16 that it is difficult to differentiate mono and bilayer

graphene as the quantity of adventitious carbon vary from one sample to the other: sometimes a

monolayer graphene sample contains more carbon than the bilayer. We thus introduced another step

before the XPS analysis consisting in an annealing of the sample under vacuum which efficiently

removes the carbon contamination. The results after annealing (on the right hand side of Figure 16) show

that mono-, bi-, tri- and four layer(s) graphene can be easily differentiated by XPS.

The XPS technique present some advantages and drawbacks compared to the usual Raman

spectroscopy:

-The drawbacks are: (i) an annealing step is necessary, (ii) XPS is a more expensive machine and more

time consuming than Raman, (iii) a minimum size of the graphene flake is required (~20-50 µm).

-The advantages are : (i) an unambiguous determination (after annealing) of the number of layers, (ii) it

is possible to determine the thickness also on chemically modified samples and on other substrates, (iii)

during the same analysis it is possible to determine the chemical state of carbon atoms (sp2, sp3) and

to analyse the impurities present on the surface of graphene.

In the future, we will improve the method (fitting of the C1s) and perform analysis on large CVD graphene

samples and on other substrates.

Figure 16: Carbon content measured by XPS on mono and few-layer graphene exfoliated on Si/SiO2

substrates.

0

5

10

15

20

25

30

35

40

45

50

After annealing

Carb

on c

onte

nt o

n Si

O2/S

i sub

stra

tes

(at%

)

Monolayer Bilayer Trilayer Four-layer

Before annealing

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